The present invention relates generally to optically pumped lasers, and more particularly to the generation of continuous wave laser radiation in the 16 micron region of the infrared spectrum using gaseous CF.sub.4.
Gas laser sources at 16 microns are an attractive alternative to diode lasers as probes of semiconductors for production and research because of their superior mode quality and consequent ability to probe dimensions which were previously unavailable due to diffraction limitations. Moreover, diode laser powers are currently limited to the milliwatt range. A more powerful 16 micron laser source would have utility as an alignment laser and master oscillator in master-oscillator power-amplifier systems, as well as for spectroscopy, including ultra-high resolution saturation spectroscopy, especially when coupled with recent developments in continuously tunable microwave shifting technology. In "CW and Improved Pulsed Operation of the 14- and 16-.mu.m CO.sub.2 Lasers" by B. L. Wexler, T. J. Manuccia and R. W. Waynant, Appl. Phys. Lett. 31, 730 (1977), the authors describe watts of laser power output in the 14 and 16 .mu.m region of the infrared spectrum using an electric discharge gas dynamic laser. This technique, which is cumbersome and complicated, does not teach the optical pumping technique of the present invention. N. V. Karlov, Yu. B. Konev, Yu. N. Petrov, A. M. Prokhorov, and O. M. Stel'makh in their journal article "Laser Based On Boron Trichloride," JETP Lett. 8, 12 (1968), describe a method for generating 12, 13 and 14 .mu.m laser radiation. Therein the authors teach the introduction of BC1.sub.3 into the discharge tube of a cw CO.sub.2 laser. The method of Karlov et al. relies on the optical pumping of a fundamental band of BC1.sub.3 leading to laser oscillation involving a difference band rather than on the optical pumping of a combination band as taught in the subject invention. Moreover, the optical pumping of Karlov et al is performed intracavity in the presence of the excited CO.sub.2 gain medium.
In "CW Optically Pumped 12-.mu.m NH.sub.3 Laser" by C. Rolland, B. K. Garside and J. Reid, Appl. Phys. Lett. 40, 655 (1982), the authors describe the use of a 30 W cw CO.sub.2 laser operating in the 9 .mu.m region to pump a ring laser cavity containing NH.sub.3 to obtain emission at 12 .mu.m. Therein the authors disclose a spectroscopic process which is very different from that of the subject invention. In particular, a strongly allowed transition in ammonia is pumped off-resonance by the CO.sub.2 pump laser, and a Raman process utilized to achieve laser oscillation off-resonance from another allowed ammonia transition. This arrangement is critical to achieving the laser oscillation since, in this manner, the large absorption possible from the lower laser level can be avoided. Further, because of low gain in their optical system Rolland et al. did not observe the counterpropagating 12 .mu.m laser beam in their ring laser configuration. However, if they were to improve their system, both beams would be present and intracavity optics would have to be employed to eliminate one of the beams. The apparatus of the present invention automatically selects one or the other of the two laser beams in a manner inherent to the Doppler effect and specialized pumping technique.
In "High-Repetition-Rate CF.sub.4 Laser" by John Telle, Opt. Lett. 7, 201 (1982), the inventor of the subject invention teaches the use of an apparatus similar in design to that of the subject invention for obtaining pulsed laser operation from CF.sub.4 using a pulsed CO.sub.2 pump laser. The spectroscopic levels employed therein are identical to those of the subject invention. Moreover, the discovery of cw laser oscillation in CF.sub.4 is mentioned on line 19 of the first column of page 202. However, there is no teaching as to what might be necessary to achieve this oscillation in optically pumped CF.sub.4. The extension of the pulsed oscillation observation to predict cw oscillation is intuitively impossible because of the complexity of the spectroscopy and relaxation phenomena. Indeed, sophisticated calculations must be undertaken to discover the equilibrium populations in the molecular levels before the appropriate experimental conditions can be determined. It should be pointed out that no cw laser oscillation has ever been predicted or observed for the spectroscopic band system utilized in the present invention. The major difficulty was the expected magnitudes of the rates of relaxation and diffusion of the three energy levels of CF.sub.4 involved in the laser oscillation. However, it was discovered that at sufficiently low gas pressures, the depletion of the ground state by the action of the pump laser is reversed by diffusion, and buildup of the lower lasing level is reversed by diffusion, both leading to unexpectedly high gain, while the population of the excited lasing level is not too adversely effected by the same diffusion process which could have resulted in severely decreased gain. The interrelationships among these quantities could not be determined except by detailed calculation. Furthermore, these interrelationships could not have been deduced from successful pulsed operation because for pulsed operation there is a time period in between pulses where the molecular system can relax to an equilibruim state totally different from the equilibrium state which exists when the pump laser is continuously operating, as in the cw oscillation situation. FIG. 4 of the above-referenced paper further reveals that the pulsed laser output maximizes at CF.sub.4 pressures of about 0.3 torr, and that the output power drops off severely at both higher and lower pressures. Calculations revealed that the optimum pressure for cw oscillation would occur at lower pressures, and it was later verified that at pressures greater than 0.1 torr of CF.sub.4, the cw oscillation output power drops precipitously as the pressure is increased. Therefore, although cw and pulsed laser oscillation are possible in an overlapping region of pressure, this condition is fortuitous and the pressures involved do not represent the optimal situation for either system. The discovery of the natural occurrence of large rotational reservoirs and reasonable diffusion times at low pressure then has led to the experimental verification of the fact that cw laser oscillation can occur in an optically pumped gas having the appropriate spectroscopic energy levels. This was previously believed to be impossible because of long-lived lower vibrational states.
In "Off-Axis Paths in Spherical Mirror Interferometers" by D. Herriott, H. Kogelnik and R. Kompfner, Appl. Opt. 3, 523 (1964), the authors describe a spherical mirror interferometer which is illuminated by an off-axis ray of light and wherein such rays retrace their paths. It is further taught that such ray paths give rise to long effective pathlengths obtainable in a small volume of active medium thereby making this configuration useful for laser amplifiers and for absorption cells. The present invention makes use of a similar design in that the cell used for the CF.sub.4 pumping requires a substantial pathlength where the pump laser and the CF.sub.4 oscillation overlap in order to provide sufficient gain to permit laser oscillation. However, Herriott et al. mention that the use of a nonconfocal spherical mirror interferometer, where off-axis paths are desired, may adversely affect the performance of an interferometer, and Herriott et al. thereby teach away from the use of such a configuration as a laser resonator. The use of this device as a resonator has been demonstrated by the success of the subject invention in achieving significant output for the optically pumped CF.sub.4 laser.
In "Continuous Wave 16 .mu.m CF.sub.4 Laser," IEEE J. Quantum Electron. QE-19, 1469 (1983) published in October, 1983, the disclosure of which is hereby incorporated by reference herein, the inventor of the subject invention describes in detail the kinetics, spectroscopy and resonator applicable to the subject invention.